Auto-tempering of hexagonal martensite in a Ti-2 wt% Mo alloy

The b c c ~ h c p phase transformation in titanium alloys on cooling can be very complex, even in relatively simple binary systems. Recent phase transformation studies by the authors [1] of a Ti -2 wt % Cr alloy have shown that a martensitic /~ ~ a ' transformation can precede the diffusional /3-+a transformation for certain cooling rates (~10°Csec-1) , in a manner identical to that previously reported for dilute Ti-Mo alloys [2-4]. This unusual phase transformation behaviour has been explained by assuming that the M s temperature lies above the "C" curve for the diffusional bcc ~ hcp transformation, as would be the case for cooling rates between CR-1 and CR-2 on the hypothetical continuous-cooling transformation diagram in Fig. 1 [2]. Also observed in the as-cooled T i 2 w t % Cr alloy were fine, plate-shaped /3 precipitates within the primary (x' martensite plates. This h c p ~ b c c reaction was attributed to "auto-tempering" of the initially supersaturated a' martensite plates during cooling to ambient temperature, and had not been reported previously for dilute Ti-Mo alloys. The purpose of the present study was to investigate the phase transformation behaviour of a T i -2 wt % Mo alloy prepared and heat treated identically to that of the previously described T i -2 wt % Cr alloy, with particular emphasis on determining tendencies for auto-tempering. The T i 2 w t % Mo alloy was prepared by triple-arc-melting high-purity titanium sponge and a Ti -50 wt % Mo master alloy. Ingot thermomechanical processing and heat treatment were as described previously [1], and resulted in chemically homogeneous specimens with a /3 grain size of about 1 mm and an oxygen level of 0.14 wt %. Structure characterization of specimens cooled from 1000°C at approximately 500, 10